OBJECTIVE FTO is the most important polygene identified for obesity. We aimed to investigate whether a variant in FTO affects type 2 diabetes risk entirely through its effect on BMI and how FTO influences BMI across adult life span.
RESEARCH DESIGN AND METHODS Through regression models, we assessed the relationship between the FTO single nucleotide polymorphisms rs9939609, type 2 diabetes, and BMI across life span in subjects from the Norwegian population-based HUNT study using cross-sectional and longitudinal perspectives. For replication and meta-analysis, we used data from the Malmö Diet and Cancer (MDC) and Malmö Preventive Project (MPP) cohorts, comprising a total sample of 41,504 Scandinavians.
RESULTS The meta-analysis revealed a highly significant association for rs9939609 with both type 2 diabetes (OR 1.13; P = 4.5 × 10−8) and the risk to develop incident type 2 diabetes (OR 1.16; P = 3.2 × 10−8). The associations remained also after correction for BMI and other anthropometric measures. Furthermore, we confirmed the strong effect on BMI (0.28 kg/m2 per risk allele; P = 2.0 × 10−26), with no heterogeneity between different age-groups. We found no differences in change of BMI over time according to rs9939609 risk alleles, neither overall (∆BMI = 0.0 [−0.05, 0.05]) nor in any individual age stratum, indicating no further weight gain attributable to FTO genotype in adults.
CONCLUSIONS We have identified that a variant in FTO alters type 2 diabetes risk partly independent of its observed effect on BMI. The additional weight gain as a result of the FTO risk variant seems to occur before adulthood, and the BMI difference remains stable thereafter.
Genomewide association studies (GWAS) have identified a strong correlation between BMI and FTO single nucleotide polymorphisms (SNPs) (1–4), and the association has been confirmed in multiple populations (reviewed in 5). The FTO risk variants are also associated with obesity-related traits (6–8). However, these effects appear to be secondary to weight increase because the associations are attenuated after adjusting for BMI (2). In contrast, we and others have found that the association with type 2 diabetes may not be completely mediated through BMI, because it remains significant after BMI correction (9). This indicates that the relationship between sequence variation in FTO and type 2 diabetes is not fully mediated through BMI or that BMI in some populations does not reveal accurate estimates of the effect of FTO on adiposity.
Various studies have investigated the effect of FTO variants on BMI and weight in a longitudinal perspective (10–18) but with diverging results. With access to extensive data from three large Scandinavian populations, through a meta-analysis approach using both cross-sectional and longitudinal data, we aimed to investigate whether the FTO risk allele affects type 2 diabetes risk after correction for BMI and whether it influences weight gain during adult life.
RESEARCH DESIGN AND METHODS
Definition of cohorts.
We studied HUNT2, a subset (aged ≥20) of a Norwegian population-based health survey (Nord-Trøndelag Health Study) (19). Our material comprised 1,740 diabetic individuals (1,543 with type 2 diabetes) and 3,856 population-based control subjects drawn from the same study population. We also had access to data on diabetes status, weight, and height from HUNT1 (1985) for 4,625 of the 5,596 subjects in HUNT2 (1995), i.e., 10-year follow-up. During these 10 years, 1,089 individuals developed type 2 diabetes. Diagnosis of diabetes was self-reported or identified by standard tests if random glucose was >8.0 mmol/L.
The Malmö Diet and Cancer (MDC) cohort (20) with baseline examinations from 1991 to 1996 consisted of 28,449 individuals. All men born between 1923 and 1945 and all women born between 1923 and 1950 from Malmö were invited. Diabetes diagnosis at baseline was self-reported or diagnosed if fasting plasma glucose was ≥7.0 mmol/L.
In the Malmö Preventive Project (MPP) cohort (21), 33,346 subjects from Malmö participated in a health screening. Men were included from 1974 to 1990, and women were included from 1980 to 1992. Eligible participants (25,000) were invited to a rescreening visit during 2002–2006. Of those invited, 16,061 nondiabetic subjects, 2,063 of whom developed type 2 diabetes during follow-up, were included in the current study. Diabetes diagnosis was taken from patient records or if fasting plasma glucose was ≥7.0 mmol/L.
The clinical characteristics of individuals from the three cohorts are shown in Table 1.
SNP selection, genotyping, and quality control.
Because the reported obesity-associated FTO SNPs are in strong to perfect linkage disequilibrium (LD) (pairwise r2 > 0.8; HapMap; CEU, release 21), we included rs9939609, reported by the first GWAS (2), as proxy. We genotyped HUNT2 subjects by MassARRAY iPLEX System (SEQUENOM, San Diego, CA). Duplicate concordance rate was 99.7% (N = 3,761). MDC individuals were genotyped using MassARRAY iPLEX System or a TaqMan assay (Applied Biosystems, Foster City, CA). Individuals (249 total) were genotyped with both methods (99.4% allelic concordance rate). A total of 8,175 individuals were genotyped twice (allelic concordance rate 99.3%). MPP subjects were genotyped by TaqMan assay with duplicate concordance rate 99.7% (N = 7,926). The final genotyping success rates were 99.7, 99.2, and 97.2%, in HUNT2, MPP, and MDC, respectively. The FTO SNP was in Hardy-Weinberg equilibrium (P value >0.05) in all cohorts.
We used logistic regression to investigate the association between type 2 diabetes and FTO genotype under an additive model. Age, sex, different combinations of BMI, waist circumference, and waist-to-hip ratio were included as covariates. In HUNT2 and MPP, follow-up measures were used unless otherwise stated. In MDC, baseline measures were used because of no available follow-up. To evaluate the risk of developing incident type 2 diabetes according to the three FTO genotypic classes, we designed a case-control study using only subjects who were healthy at baseline. Those developing type 2 diabetes during follow-up were defined as cases and the rest as control subjects. We used logistic regression models corrected for sex, baseline age, and BMI. In a second step, we included the change in BMI over time (∆BMI) as an additional cofactor.
To analyze the association between FTO genotype and BMI, we used linear regression models assuming additive effects of allele dosage with adjustment for age, sex, and diabetes status. Logarithmic transformation of BMI values did not change the results; thus, only results using nontransformed BMI are presented. To assess whether the allele-wise increase in BMI differed across different adult ages, we performed individual analysis in every 10-year age stratum in the cross-sectional and longitudinal datasets. Finally, meta-analysis was performed to combine the regression coefficients (per allele change in BMI) with their standard errors from within the three cohorts and for each specific age-group. Interstudy heterogeneity and heterogeneity between different age-groups were estimated using Cochran’s Q test and the I2 statistic. Overall estimates were calculated using a fixed-effect model with inverse variance.
We performed statistical analyses by PLINK (22) and Stata SE v10.0 (Stata, Brownsville, TX) or SPSS (version 18; SPSS Inc., Chicago, IL). Meta-analysis statistics and plots were produced using the METAN module (23) developed for Stata and with GWAMA (Genome-Wide Association Meta-Analysis) software (24).
Relationship among FTO, type 2 diabetes, and obesity-related quantitative traits across life span in HUNT.
After correction for age and sex, we observed a strong association with type 2 diabetes for rs9939609 in HUNT2. This association remained significant after correction for BMI (OR 1.19 [95%CI 1.09–1.30]; P = 1.8 × 10−5). The FTO variant also conferred an increased risk for type 2 diabetes after adjustment for waist circumference and waist-to-hip ratio. These results suggested that rs9939609 has an effect on the risk of type 2 diabetes, an effect that cannot be entirely explained through increased BMI or central obesity.
Using a cross-sectional design, we observed that the FTO-associated allele-wise increase in BMI persisted at the same level throughout life (Supplementary Fig. 1). In addition, rs9939609 × age interactions on obesity-related traits were all nonsignificant (Supplementary Table 1), suggesting that changes in these traits by age were not dependent on the individual’s FTO genotype.
Next, we studied longitudinal change in the association between FTO and BMI during 10-year follow-up from HUNT1 to HUNT2. The FTO variant showed an association with all obesity-related quantitative traits (Supplementary Table 1). There was, however, no association between rs9939609 and ∆BMI between 1985 and 1995. This suggested that the FTO-associated relative difference of BMI is established before adulthood and then remains stable.
Confirmation of the findings from HUNT–meta-analysis in 41,504 Scandinavians.
Clinical characteristics of individuals from the three different cohorts are presented in Table 1. The minor allele frequencies of rs9939609 in nondiabetic individuals were 0.42, 0.41, and 0.41 in HUNT2, MPP, and MDC, respectively.
The meta-analysis demonstrated that the association between rs9939609 and type 2 diabetes was strong after adjustment for age and sex (OR 1.13 [95%CI 1.08–1.19]; P = 4.5 × 10−8) and remained significant after BMI correction (OR 1.09 [95%CI 1.04–1.15]; P = 1.2 × 10−4; Fig. 1A and B). Correction for waist-to-hip ratio or waist circumference instead of BMI did not change the results (Supplementary Fig. 2A–C). To further elucidate whether rs9939609 exerts an effect on type 2 diabetes independently of BMI, we evaluated the risk to develop incident type 2 diabetes according to FTO genotype during follow-up. As shown in Supplementary Fig. 3A–C, the association remained similar for incident type 2 diabetes after correction for sex and baseline age and BMI (OR 1.12 [95%CI 1.05–1.18]; P = 1.1 × 10−4) and after correction also for ∆BMI (OR 1.11 [95%CI 1.05–1.18]; P = 1.5 × 10−4).
The meta-analysis of the FTO-associated allele-wise effect on BMI using cross-sectional data confirmed the strong effect of the FTO SNP on BMI (0.28 kg/m2 per risk allele [P = 2.0 × 10−26]; Fig. 2A). Furthermore, we detected no heterogeneity in the effect sizes for the FTO risk allele between the different age-groups (Fig. 2B). Finally, Fig. 3 shows the linear regression summary results between rs9939609 and ∆BMI for all HUNT and MPP individuals for whom longitudinal data were available. There was no significant difference in ∆BMI according to overall number of rs9939609 risk alleles (∆BMI = 0.0 [−0.05, 0.05]) or in any individual age stratum (Fig. 3B). Hence, the FTO-associated effect on BMI seems to establish relatively early in life, and the relative BMI difference remains stable across adult life.
To our knowledge, this is the largest study investigating the effect of FTO sequence variants on type 2 diabetes and BMI across the whole range of adult ages and in a longitudinal perspective. In 41,504 Scandinavians, we demonstrate that a common variant of FTO does not mediate type 2 diabetes risk entirely through its influence on BMI. Although our findings are comparable with some earlier studies (25–27), they contrast previous results reported in most populations studied to date, including Europeans (1–3,8). Reasons for the diverging results could be differences in selection or recruitment of cases and control subjects between studies, differences in undetected key effects at early age, or population-specific environmental factors that may interact with the way FTO works to influence the risk of type 2 diabetes. In an attempt to capture the complex relationship between FTO, BMI, and type 2 diabetes during the life course, we performed an analysis on incident type 2 diabetes. The results remained similar in the longitudinal study both when we controlled for BMI at baseline (before diabetes was diagnosed), ∆BMI, or waist circumference and/or waist-to-hip ratio as covariates in the regression analyses. None of the covariates alone or in combination with BMI changed our results notably. FTO still conferred an increased risk for type 2 diabetes.
How sequence variation in FTO could possibly affect type 2 diabetes risk in other forms than through increased adiposity remains elusive. No associations have been reported between FTO SNPs and glucose tolerance or insulin sensitivity. A link between SNPs in FTO and altered lipid profiles has been suggested (6,9), but we could not confirm this in our meta-analysis (Supplementary Table 2). It has been suggested that rs9939609 affects the primary allelic FTO transcript levels (28), and correlations have been observed in peripheral tissues between BMI of tissue donors and FTO mRNA expression levels (29). It is noteworthy that three recent FTO expression studies support a potential role in type 2 diabetes independently of BMI. One study found no association between FTO expression and BMI in islet cells (30). Another study reported an inverse correlation between Fto mRNA and glucose in mice after correction for body weight (31). Finally, a third study found an increase of FTO mRNA and protein levels in muscle from type 2 diabetic patients compared with healthy lean control subjects or BMI-matched obese nondiabetic individuals (32). The latter also suggests that increased FTO expression in type 2 diabetic patients contributes to reduced mitochondria oxidative capacities, lipid accumulation, and oxidative stress, all associated with type 2 diabetes. It is also possible that the rs9939609 SNP (or a SNP in strong LD) affects another gene in the region, which has the potential to alter type 2 diabetes risk independently of BMI (33).
The association between FTO sequence variants and BMI is not established at birth (2,34) but seems to evolve gradually before adulthood (2,35,36). It is not clear how FTO genotype affects BMI after adolescence and develops during the life course (10–18), although a recent longitudinal Finnish study suggests that the effect may continue into adulthood since they found an association between rs9939609 and BMI at age 31, which could not be explained by the BMI at age 14 (18). Using cross-sectional and longitudinal designs, we identified in the three Scandinavian populations that the relative difference in mean BMI among individuals with different rs9939609 genotypes remains surprisingly stable across all adult ages. Hence, because our study primarily comprised individuals that were above 30 years of age (98.7%), current evidence suggest that the FTO variant increases BMI in the first 2 to 3 decades of life, and from then on the BMI difference between the genotypes becomes more or less constant throughout life. Nevertheless, it remains to be seen whether other relevant factors such as diet and physical activity may interact and modify the susceptibility to obesity by the FTO variants during the life course (37–39).
In summary, we have replicated that a common variant in the FTO gene alters type 2 diabetes risk but find that this association is partly independent of the effect on BMI. Our data further demonstrate that the weight gain as a result of the FTO risk variant occurs during youth and that the BMI difference according to the FTO genotype persists at the same level throughout life, setting the threshold for BMI.
The HUNT study was supported in part by funds from the University of Bergen, Haukeland University Hospital, Helse Vest, Innovest, and the Research Council of Norway. Genotyping was in part provided by the CIGENE technology platform (Ås, Norway), which is supported by the Functional Genomics Programme (FUGE) of the Research Council of Norway. HUNT is a collaboration between the HUNT Research Center at the Norwegian University of Science and Technology, Levanger; the Norwegian Institute for Public Health; and the Nord-Trøndelag County Council. The diabetes part of HUNT was partly supported by funds from GlaxoSmithKline Norway and the Norwegian Diabetes Association. The MDC study was supported by project grants from the Swedish Research Council, the European Foundation for the Study of Diabetes, the Novo Nordisk and Albert Påhlsson Foundations, a Linnaeus grant to the Lund University Diabetes Centre, and the Knut and Alice Wallenberg Foundation. The MPP study was supported by grants from the Swedish Research Council (including Linné grant 31475113580), the Heart and Lung Foundation, the Diabetes Research Society, a Nordic Center of Excellence Grant in Disease Genetics, the Diabetes Program at the Lund University, the European Foundation for the Study of Diabetes, the Påhlsson Foundation, the Craaford Foundation, the Novo Nordisk Foundation, the European Network of Genomic and Genetic Epidemiology, and the Wallenberg Foundation. L.G. has been a consultant for and served on advisory boards for sanofi-aventis, GlaxoSmithKline, Novartis, Merck, Tethys Bioscience, and Xoma and received lecture fees from Eli Lilly and Novartis. No other potential conflicts of interest relevant to this article were reported.
J.K.H. and S.J. designed the study, wrote the manuscript, researched data, contributed to the discussion, and reviewed and edited the manuscript. E.S. and A.J. researched data and reviewed and edited the manuscript. R.T.L. contributed to the discussion and reviewed and edited the manuscript. C.G.P.P. researched data and contributed to the discussion. P.M.N. contributed to the discussion and reviewed and edited the manuscript. G.R. researched data. K.M. researched data and reviewed and edited the manuscript. K.H. researched data, contributed to the discussion, and reviewed and edited the manuscript. O.M. reviewed and edited the manuscript. L.G. contributed to the discussion and reviewed and edited the manuscript. V.L. researched data and reviewed and edited the manuscript. A.M. designed the study, contributed to the discussion, and reviewed and edited the manuscript. M.O.-M. researched data and reviewed and edited the manuscript. P.R.N. designed the study, contributed to the discussion, and reviewed and edited the manuscript.
This article contains Supplementary Data online at http://diabetes.diabetesjournals.org/lookup/suppl/doi:10.2337/db10-1340/-/DC1.
- Received September 20, 2010.
- Accepted February 17, 2011.
- © 2011 by the American Diabetes Association.
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